1. Field of the Invention
This invention relates to a surface treatment method for tin-free steel. More particularly, the invention relates to a surface treatment method to improve the adhesiveness of a coating formed by coating with paint and baking on the surface of a steel sheet having a hydrated chromic oxide on a chromium plate layer.
2. Description of the Prior Art
As the tin-free steel recently developed as a material for food cans is excellent in adhesion to organic polymer materials such as paints and adhesive, the so-called three-piece bonded cans whose shells are bonded using the above can material are manufactured and put of the market. In case the content of the bonded can is a carbonated beverage, beer or the like, high temperature is not applied to the bonded portion of the can before and after the content is packed in the can. Therefore, it is seldom that the bonding strength decreases and that damage is caused to the can shell. Further, the bonding strength scarcely decreases when the can after the packing is stored at room temperature for a long time.
Lately, however, there is a tendency to use the bonded cans for other contents which may need high-temperature packing and high-temperature sterilization after the packing. In this case, if the bonded portion of the bonded can is exposed to the content containing water of high temperature, water molecules filtration through the coating layer severely promote deterioration of the bonded portion, which causes the bonded portion of the can shell to be fractured in a short time during or after the packing, thereby giving rise to leakage or broken shell.
A cross-section of the bonded portion of a typical bonded can is shown in FIG. 1, in which the reference numberal 1 designates tin-free steel, 2 designates a coating layer and 3 designates a adhesive. Through the strength of such bonded portion depends on the tensile strength of coating layer 2 and adhesive 3, the locations of fracture which are the most important porblem are between tin-free steel 1 and coating layer 2, between coating layer 2 and adhesive 3 and between the chromium plate layer and the hydrated chromic oxide in tin-free steel 1.
The bond between coating layer 2 and the surface of tin-free steel 1 is thought to be due to the hydrogen bond formed between the OH group in the hydrated chromic oxide and the --OH group of an epoxy-phenol paint most generally used for coatings. This hydrogen bond is higher in binding strength than the physical adsorption most generally caused between coatings and metallic surfaces. However, if the bonded portion is brought into contact with a water solution containing polar substances such as urea and the like, the bond is caused to be broken. In particular, the higher the temperature of such water solution, the more the quantity filtration through the coating and bonding agent increases and the more the speed of breaking the bond increases. Therefore, the bonded portion disadvantageously is caused to be fractured between coating 2 and tin-free steel 1. Moreover, even at lower ambient temperatures, the bonding strength inevitably decreases below the initial value with the lapse of time.
It is an object of this invention to provide a surface treatment method for tin-free steel being suitable for bonded cans, which permits to prevent the fracture to be caused in a short time to the bonded portion of the bonded can after subjected to high-temperature treatment such as high-temperature packing and high-temperature sterilization after the content having been packed in the can.
It is another object of the invention to provide a surface treatment method for tin-free steel being suitable for bonded cans, which permits the bonded can to maintain the initial bonding strength during long-term storage after the content having been packed in the can and the can subjected to high-temperature treatment.